Network Working Group P. Eronen
Request for Comments: 4739 Nokia
Category: Experimental J. Korhonen
TeliaSonera
November 2006
Multiple Authentication Exchangesin the Internet Key Exchange (IKEv2) Protocol
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2006).
Abstract
The Internet Key Exchange (IKEv2) protocol supports several
mechanisms for authenticating the parties, including signatures with
public-key certificates, shared secrets, and Extensible
Authentication Protocol (EAP) methods. Currently, each endpoint uses
only one of these mechanisms to authenticate itself. This document
specifies an extension to IKEv2 that allows the use of multiple
authentication exchanges, using either different mechanisms or the
same mechanism. This extension allows, for instance, performing
certificate-based authentication of the client host followed by an
EAP authentication of the user. When backend authentication servers
are used, they can belong to different administrative domains, such
as the network access provider and the service provider.
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RFC 4739 Multiple Auth. Exchanges in IKEv2 November 20061. Introduction
IKEv2 [IKEv2] supports several mechanisms for parties involved in the
IKE_SA (IKE security association). These include signatures with
public-key certificates, shared secrets, and Extensible
Authentication Protocol (EAP) methods.
Currently, each endpoint uses only one of these mechanisms to
authenticate itself. However, there are scenarios where making the
authorization decision in IKEv2 (whether to allow access or not)
requires using several of these methods.
For instance, it may be necessary to authenticate both the host
(machine) requesting access, and the user currently using the host.
These two authentications would use two separate sets of credentials
(such as certificates and associated private keys) and might even use
different authentication mechanisms.
To take another example, when an operator is hosting a Virtual
Private Network (VPN) gateway service for a third party, it may be
necessary to authenticate the client to both the operator (for
billing purposes) and the third party's Authentication,
Authorization, and Accounting (AAA) server (for authorizing access to
the third party's internal network).
This document specifies an extension to IKEv2 that allows the use of
multiple authentication exchanges, using either different mechanisms
or the same mechanism. This extension allows, for instance,
performing certificate-based authentication of the client host
followed by an EAP authentication of the user.
Each authentication exchange requiring communication with backend AAA
servers may be directed to different backend AAA servers, located
even in different administrative domains. However, details of the
communication between the IKEv2 gateway and the backend
authentication servers are beyond the scope of this document. In
particular, this document does not specify any changes to existing
AAA protocols, and it does not require the use of any particular AAA
protocol.
In case of several EAP authentications, it is important to notice
that they are not a "sequence" (as described in Section 2.1 of
[EAP]), but separate independent EAP conversations, which are usually
also terminated in different EAP servers. Multiple authentication
methods within a single EAP conversation are still prohibited as
described in Section 2.1 of [EAP]. Using multiple independent EAP
conversations is similar to the separate Network Access Provider
(NAP) and Internet Service Provider (ISP) authentication exchanges
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RFC 4739 Multiple Auth. Exchanges in IKEv2 November 20061.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
The terms and abbreviations "authenticator", "backend authentication
server", "EAP server", and "peer" in this document are to be
interpreted as described in [EAP].
When messages containing IKEv2 payloads are described, optional
payloads are shown in brackets (for instance, "[FOO]"), and a plus
sign indicates that a payload can be repeated one or more times (for
instance, "FOO+").
2. Solution2.1. Solution Overview
The peers announce support for this IKEv2 extension by including a
MULTIPLE_AUTH_SUPPORTED notification in the IKE_SA_INIT response
(responder) and the first IKE_AUTH request (initiator).
If both peers support this extension, either of them can announce
that it wishes to have a second authentication by including an
ANOTHER_AUTH_FOLLOWS notification in any IKE_AUTH message that
contains an AUTH payload. This indicates that the peer sending the
ANOTHER_AUTH_FOLLOWS wishes to authenticate another set of
credentials to the other peer. The next IKE_AUTH message sent by
this peer will contain a second identity payload (IDi or IDr) and
starts another authentication exchange. The IKE_AUTH phase is
considered successful only if all the individual authentication
exchanges complete successfully.
It is assumed that both peers know what credentials they want to
present; there is no negotiation about, for instance, what type of
authentication is to be done. As in IKEv2, EAP-based authentication
is always requested by the initiator (by omitting the AUTH payload).
The AUTH payloads are calculated as specified in [IKEv2] Sections
2.15 and 2.16, where IDi' refers to the latest IDi payload sent by
the initiator, and IDr' refers to the latest IDr payload sent by the
responder. If EAP methods that do not generate shared keys are used,
it is possible that several AUTH payloads with identical contents are
sent. When such EAP methods are used, the purpose of the AUTH
payload is simply to delimit the authentication exchanges, and ensure
that the IKE_SA_INIT request/response messages were not modified.
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RFC 4739 Multiple Auth. Exchanges in IKEv2 November 20063. Payload Formats3.1. MULTIPLE_AUTH_SUPPORTED Notify Payload
The MULTIPLE_AUTH_SUPPORTED notification is included in the
IKE_SA_INIT response or the first IKE_AUTH request to indicate that
the peer supports this specification. The Notify Message Type is
MULTIPLE_AUTH_SUPPORTED (16404). The Protocol ID and SPI Size fields
MUST be set to zero, and there is no data associated with this Notify
type.
3.2. ANOTHER_AUTH_FOLLOWS Notify Payload
The ANOTHER_AUTH_FOLLOWS notification payload is included in an
IKE_AUTH message containing an AUTH payload to indicate that the peer
wants to continue with another authentication exchange. The Notify
Message Type is ANOTHER_AUTH_FOLLOWS (16405). The Protocol ID and
SPI Size fields MUST be set to zero, and there is no data associated
with this Notify type.
4. IANA Considerations
This document defines two new IKEv2 notifications,
MULTIPLE_AUTH_SUPPORTED and ANOTHER_AUTH_FOLLOWS, whose values are
allocated from the "IKEv2 Notify Message Types" namespace defined in
[IKEv2].
This document does not define any new namespaces to be managed by
IANA.
5. Security Considerations
Security considerations for IKEv2 are discussed in [IKEv2]. The
reader is encouraged to pay special attention to considerations
relating to the use of EAP methods that do not generate shared keys.
However, the use of multiple authentication exchanges results in at
least one new security consideration.
In normal IKEv2, the responder authenticates the initiator before
revealing its identity (except when EAP is used). When multiple
authentication exchanges are used to authenticate the initiator, the
responder has to reveal its identity before all of the initiator
authentication exchanges have been completed.
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RFC 4739 Multiple Auth. Exchanges in IKEv2 November 2006
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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